Fluorine-containing surfactants represented by the general formula RfCOOM (Rf: a perfluoroalkyl group, and M: an alkali metal, an ammonium group, or a hydrogen atom) have been so far widely used in the emulsion polymerization of fluorine-containing monomers, among which perfluorooctanoic acid (or its salt) C7F15COOM as one of typical surfactants in well known as the most distinguished one, because of its good monomer emulsifiability and latex stability, and easy washability following the salting-out operation.
However, it has been so far clarified that the perfluorinated compounds are hard to decompose in the natural environments, and perfluorooctanoic acid (or its salt) as a typical perfluorinated C8 compound considerably remains in human bodies. It is expectable that an environmental decomposability can be given by forming hydrogenated portions in the perfluorinated hydrophobic groups of the surfactant compound. To decrease the amount of remaining the decomposates in the environments, it is also a desirable that the hydrophobic group must contain a hydrocarbon chain so as to limit a succession of the perfluorinated carbon atoms to less than 8 carbon atoms such as RfCmH2mCnF2n— (Rf: a perfluoroalkyl group having 1 to 7 carbon atoms, n: an integer of 1 to 7, and m: an integer of 1 or more).
As to the behavior of surfactants at the time of emulsion polymerization reaction, it has been so far proposed to use a perfluoroalkylethane sulfonic acid (or its salt) F(CF2CF2)nCH2CH2SO3M (M: a monovalent cation, and n: 2-8) as a dispersing agent at the time of tetrafluoroethylene copolymerization, though the dispersing agent compound undergoes chain transfer at the time of polymerization reaction, so that the lowering in the molecular weight of the resulting tetrafluoroethylene copolymers is quite unavoidable.    Patent Literature 1: U.S. Pat. No. 4,380,618
It has been also so far disclosed that partially fluorinated, hydrogen-containing fluorine-based surfactants represented by the general formula Rf(CH2)mRf′ COOM (Rf: a perfluoroalkyl group or a perfluoroalkoxy group having 3 to 8 carbon atoms, Rf′: a linear or branched perfluoroalkylene group having 1 to 4 carbon atoms, M: NH4, Li, Na, K or H; and m: an integer of 1 to 3) are used in the polymerization of fluorinated monomers as a hydrogen-containing fluorine-based surfactant having a low chain transferability at the time of polymerization reaction to increase the molecular weight in the homopolymerization of tetrafluoroethylene.    Patent Literature 2: JP-A-10-212261
Such hydrogen-containing fluorine-based surfactant compounds are synthesized by subjecting fluoroalkylethylene or fluoroalkoxyethylene represented by the general formula Rf(CH2)xCH═CX2 (X: H or F) to reaction with an iodized ester represented by the general formula IRf′ COOR to make iodine-containing intermediates represented by the general formula:Rf(CH2)xCHI(CH2)yRf′COOR(y:0 or 1),followed by reduction to form esters represented by the general formula:Rf(CH2)mRf′COOR.and further followed by hydrolysis with a base and by neutralization with an acid. In a series of these processes, the substrate is inevitably exposed to a basic environment in the alkali hydrolysis step, so that side reactions such as formation of double bonds, etc. by the dehydrofluorination reaction are liable to take place.
When the hydrogen-containing fluorine-based surfactant is used as an emulsifying agent in the polymerization reaction of fluorine-containing monomers, the monomer emulsificability and latex stability are not better than those by the emulsifying agent of perfluorooctanoic acid (or its salt). Furthermore, when that surfactant is used as an emusifying agent in the homopolymerization or copolymerization reaction of vinylidene fluoride, the micelles solubility of vinylidene fluoride as monomers is so low that the polymerization reaction is not only much retarded, but also the latex stability of the resulting polymers is so low that sometimes the polymers are liable to deposit during the polymerization reaction.